1. Field of the Invention
The present invention relates to a phosphor (photo-luminescent) material based authentication system and, more particularly, to a method of authenticating a photo-luminescent security marking, an apparatus for authenticating a photo-luminescent security marking and a photo-luminescent security marking.
2. Description of the Related Art
Authentication is the act of establishing or confirming something as being genuine, that is, claims made about the thing are true. Authenticating an article or document usually involves confirming its provenance using one or more authentication factors. Articles or documents will often include one or more security markings or security devices in order to authenticate the article/document. Examples of security markings/devices include: watermarks in documents, magnetic print on personal checks, micro-printing on banknotes, antibodies in oil and gas, molecular recognition agents in spirits and liquors, thermal transfer inks and color shifting pigments on packaging, holograms on credit cards, photo-luminescent security markings, bar codes, electronic tagging devices, invisible markers on pharmaceuticals capsules etc.
A study sponsored by the World Economic Forum (WEF) estimates the rate of counterfeiting goods such as pharmaceuticals, tobacco, luxury goods, electronics, clothing, and other items is between 7 and 9% of global trade and totals more than $700 billion per annum. The World Health Organization (WHO) and FDA (US Food and Drug Administration) estimate that anything between 5 and 8% of pharmaceutical and related products are counterfeit. In addition to jeopardizing people's health, counterfeiting results in a loss of revenue of the order of $30 million a year.
Due to continuing improvements in color printing and image scanning technologies the counterfeiting of documents, such as for example, bank notes, identification papers, passports, drivers' licenses, admission tickets is increasing. Additionally, counterfeiting of credit, debit and security cards is increasing due to the widespread availability of computers and card readers. Brand protection has become a huge strategic issue due to globalization of trade, organized crime and the high profits with often low penalties associated with counterfeiting.
To combat counterfeiting and safeguard a brand, a variety of security technologies have been developed including RFID (Radio Frequency Identification), holography, stable isotopic markers, molecular recognition markers, thermo-chromic materials, color shift inks, tags with visible and invisible fluorescent dyes, and phosphor material based markings.
Security markings that utilize a phosphor material offer a number of potential authentication factors including excitation wavelength, emission spectra with unique signatures covering ultra violet (UV) to near infrared (NIR), peak intensity, peak wavelength, decay time or interval, rise and fall of particular peaks in the spectra. To verify the authenticity of an article or document it is known to provide a phosphor security marking on the article/document. The authenticity of the article/document is verified by irradiating the article with high energy radiation from a source such as a UV lamp and measuring the intensity of UV light reflected from the object and the amount of fluorescent light generated by the phosphor security marking. If the optical characteristics (reflection and emission) are within prescribed limits, then the article is verified as being genuine.
Phosphor (photo-luminescent) materials can be characterized into two categories i) down-converting and ii) up-converting. In the case of down-converting photo-luminescent materials, the material is excited with high energy photons such as UV or high ionizing radiations such as X-rays or γ-rays and emit light in the visible or NIR spectral regions. Conversely up-converting, or anti-Stokes, photo-luminescent materials absorb two or more low energy photons such as NIR, particularly 980 nm radiation, and emit a single high energy photon of shorter wavelength (e.g. visible light) by a process of summing IR photons.
Most UV excitable down-converting phosphors, hereinafter termed UV phosphors, can be excited by 254 nm or 365 nm lines from a low pressure mercury vapor lamp. By analyzing the emission (photo luminescence) spectra, the identity of the phosphor can be determined. UV phosphors were extensively used in security markers during the 1980s and 1990s. A drawback of UV phosphors is that most of them fluoresce strongly on paper based substrates and this can reduce the effectiveness of the fluorescent marking. A further problem with UV phosphors is that the UV excitation radiation is harmful to the human eye. From a safety perspective phosphors that can be excited by an excitation radiation of wavelength within the visible part of the electromagnetic spectrum, that is 380 to 700 nm, are considered to be ideal.
The up-conversion process relies on one absorber Yb3+ (ytterbium) and typically one or more of three different emitters in blue Tm3+ (thulium), green Er3+ (erbium) or red Yb3+ regions. Absorption of a first infrared photon promotes a rare earth ion into a relatively long-lived first excited state. If a subsequent infrared photon encounters this excited ion it may, depending on the absorption probability, be absorbed, thereby promoting the ion into a second higher excited state. Transition of the ion from the second state to the ground state results in the emission of a photon of higher energy (i.e. of shorter wavelength). Since up-converting phosphor materials rely on the absorbance of Yb3+ each of them can be excited by a 980 nm (IR) laser diode. An example of an up-converting phosphor is a Eu3+, Yb3+ activated yttrium oxysulfide Y2O2S:Er,Yb. Other examples of IR (980 nm) excitable phosphors are Yb3+, Tm3+ activated gadolinium oxysulfide as are described in U.S. Pat. No. 6,686,074 and U.S. Pat. No. 6,841,092.
A problem with up-converting phosphors is that due to their relatively low efficiency they need to be excited by high intensity IR radiation generated by a laser and such radiation is harmful to the human eye. To reduce the likelihood of eye damage WO 2000/60527 teaches exciting an up-converting phosphor material using an NIR laser that is operated in a pulsed mode of operation such that the pulses have a peak power sufficient to induce a detectable emission in the phosphor material and the pulse repetition frequency and duration are selected such that the mean power of the laser emission is low enough not to produce eye damage.
Inks containing UV phosphors are readily available in the security industry at a relatively low cost. However, due to the widespread use of such inks counterfeiters have become knowledgeable about various inks and their use. Moreover, counterfeiters are able to reproduce or procure the same ink with UV phosphors that matches the characteristics of the ink with phosphor from the product sample, and apply the same on the counterfeit products. To overcome this problem, ink with up-converting phosphors that are excitable by light in the IR part of the spectrum ranging from 800 nm to 1600 nm have been developed. U.S. Pat. No. 5,766,324 describes an IR security ink that comprises an IR phosphor combined with a black colorant which does not absorb light in the IR wavelength range.
U.S. Pat. No. 7,030,371 discloses a method of measuring the luminescence characteristic of a luminescent security marking in which the luminescence emission is measured during specific time intervals either during or after exposure to the excitation radiation. The intensity value measured during one time interval is subtracted from the intensity value measured during the other time interval with the result of the subtraction being representative of light emitted from the luminescent security marking. The duration of one time interval is shorter than 25% of the exposure time to the excitation radiation.
U.S. Pat. No. 5,331,140 discloses a bar code reading system for reading a fluorescent bar code. The bar code is irradiated with radiation that is sine wave or square wave modulated at two frequencies that are not harmonically related and detecting radiation emitted by the bar code at frequencies corresponding to the sum and difference of the two modulation frequencies.
A need exists for an inexpensive authentication system that offers a high level of security and which can be verified using a portable authentication device.